1. Field of the Invention
The present invention relates to a thermal-dye transfer type recording sheet, and more particularly to a recording sheet for a thermal-dye transfer type recording, in which a coloring material layer containing sublimable dye is provided on a base sheet, the coloring material layer is brought into contact with a recording sheet and the dye is transferred to the recording sheet by heating with a thermal head or the like.
2. Prior Art
In the thermal recording methods there is a widely adopted method in which a heat-sensitive paper having thereon a recording layer to be colored under heating by a physical or chemical change is brought into contact with a thermal head and then a record of a desirable color is obtained on the heat-sensitive recording paper. The heat-sensitive recording method, however, is defective in that coloration or contamination is readily caused in a heat-sensitive recording paper because of pressure or heat unavoidably being applied to the heat-sensitive recording paper during storage or during handling and in that a highly resolved multichromatic recording is difficult to achieve technically.
As a means for overcoming the above defects of the conventional heat-sensitive recording method, there has been proposed a method as disclosed in the Japanese patent application Laid-Open Specification No. 15,446/76. It discloses a substrate, such as paper or resin film, coated with a coloring material which is solid or semi-solid at room temperatures, the coloring material coated on the substrate is brought into contact with a recording sheet and the coloring material on the substrate is selectively transferred to the recording sheet by heating by a thermal head to perform recording.
As such recording method, there can be used a wet system and a dry system. The wet system includes melting and softening the binder in the coloring material layer and adhering and transfering binder with the dye to the recording sheet, in thermal transfer recording. In the dry system, sublimable dye is used in the coloring material layer, and the adsorption on the recording sheet is carried out by sublimating the dye.
As the coloring material layer in the dry system, there is used a substance which is prepared by kneading a binder and sublimable dye having a sublimation temperature of 60.degree. to 300.degree. C.
Such sublimable dye having a sublimation temperature of 60.degree. to 300.degree. C. are, for example, disperse dyes of the nitro, azo, quinolipe and anthraquinone types. The transfer of the dye to the recording sheet does not occur at usual temperatures even when the coloring material layer is brought into contact with the recording sheet, but the transfer of the dye occurs, first when the coloring material layer is heated to 60.degree.-500.degree. C. and then the dye is sublimated. In each case, conventional plain paper can fundamentally be used.
In contrast to the wet system in which the color material layer itself is transferred, the dry system has a feature that the clearness and color density of the recording image depend on the degree of the adsorption or fixation of sublimable dye to the recording sheet surface. Accordingly, when conventional plain paper is used in the dry system in which a sublimable dye having poor affinity to fiber is applied, excellent color density cannot be obtained.
The Applicant noted in the Japanese patent application No. 182,894/1980 that the recording sheet with a coating layer comprising a saturated polyester or a mixture of saturated polyester and polyvinyl pyrrolidone provides excellent color density.